1. Technical Field
A method of fabricating a NAND type flash memory device is disclosed in which an increase in a thickness of wall oxide films due to a “smiling” effect of the tunnel oxide films formed by a wall oxidization process is prevented. To prevent the wall oxide film thickness increase, a trench formation process and a wall oxide film formation process are performed separately depending on the pattern density, and the wall oxide films are formed with different thicknesses.
2. Description of the Related Art
The most important factor to be considered in the process of forming an isolation film in a NAND-type flash memory device of below 0.07 μm may include film quality of the tunnel oxide film against a floating gate and an alignment method. For this purpose, according to the prior art, an underlying gate profile is first secured using a polysilicon film and an element isolation process is then performed to secure the film quality of the tunnel oxide film. An isolation film and a floating gate are formed in a self-aligned manner.
This method of fabricating the flash memory device will be briefly described below. A tunnel oxide film, a polysilicon film and a hard mask film are sequentially formed on a semiconductor substrate. The hard mask film is patterned using a photolithography process using an isolation mask and an etch process. Next, the polysilicon film and the tunnel oxide film are etched using the patterned hard mask film as a mask and the semiconductor substrate is then etched to a given depth, thus forming a trench. After a wall oxide film and a liner oxide film are formed in the trench, the trench is filled with an oxide film, thus forming an isolation film. Thereafter, after the hard mask film is removed, a polysilicon film is formed and is then patterned, thereby forming a floating gate.
In the aforementioned process, the trench formation process and the wall oxide film formation process are performed in a cell region and a peripheral circuit region at the same time regardless of the pattern density. Furthermore, the wall oxidization process is one of processes which has to be performed appropriately and sufficiently in order to improve ISB fail and hump characteristics as well as remove the leakage current by surface damage of the semiconductor substrate in a dry etch for forming the trench.
Accordingly, the wall oxide film must be formed in a sufficient thickness on the basis of a region where the pattern density is low. If the wall oxide film is formed on the basis of the region where the pattern density is low, more particularly, the peripheral circuit region having a high gate size, however, a side “smiling” effect of the tunnel oxide film is generated by the wall oxidization process in the region where the pattern density is dense, or more particularly, the cell region where the size of the gate become small. Therefore, there are problems in that a thickness of the tunnel oxide film increases and the film quality of the tunnel oxide film is degraded.
This situation becomes more severe during an annealing process and a re-oxidization process after a dielectric film is formed by a subsequent process. Increases in the thickness of the tunnel oxide film can cause a failure in the read and write operations which are the most basic device operations of a NAND-type flash memory device.
An increase in the thickness of the tunnel oxide film by the thermal process has a greater influence on the device when the size of the gate is smaller. Furthermore, if the size of the floating gate is smaller than that of the active region, the thickness of the tunnel oxide film is further increased as a tunneling effect is combined with the smiling effect.